High speed digital signaling apparatus and method using reflected signals to increase total delivered current

ABSTRACT

A signaling apparatus and method are described that use reflected signals to increase the total current delivered to a receiver. Dynamic source-side transmission line termination control is employed to generate reflected signals that constructively add to a nonreflected signal to enhance the signal at the receiver. Switching controls selectively connect and disconnect the transmission line source-side termination resistors to either provide signal termination or remove it. Driver designs using either voltage or current sources for use in signaling systems (including, for example, magnetic storage devices with inductive coil based write heads) are described.

FIELD OF THE INVENTION

This invention relates to high speed signaling systems usingpre-emphasis of the transmitted signal.

BACKGROUND

There are many applications in which it is desirable to enhancetransition speed of a signal that is transmitted on a transmission lineto a transducer or receiver. It is also important to provide thisenhancement while expending the least amount of power, or no extra powerat all. In the following, the particular example of a current deliveredthe write coil of a magnetic data storage device will be used as anillustrative example, which also includes the additional challenge ofmaintaining the write current level in the coil while lowering powerdissipation in the system. The current invention however has broaderapplication as will be appreciated by those of skill in the art.

Magnetic data storage devices include front-end circuits 10 (see FIG. 1)that read and write the data to the storage media, such as used in harddisk drive (HDD) storage devices, typically include one or more pairs ofmagnetic transducers 23, 24 for reading and writing magnetic transitionsin magnetic media. The slider 13 includes the read and write transducerelements. The read transducer element 23 and write transducer element 24are also called the read head and write head respectively. The slider 13is mounted on an actuator arm (not shown) which mechanically positionsthe transducers over selected tracks on the magnetic media on rotatingdisks (not shown). The electrical signals to and from the read and writetransducer elements are processed by appropriate electronic circuitry inread amplifier 21 and write driver 22 which are connected throughelectrically conductive paths 25A, 25B, 26A, 26B. The read/write channel20 reads data from the read amplifier 21 and supplies data signals towrite driver 22.

The write transducer 24 writes digital information on the rotating diskmedia on the disk by creating magnetic flux reversals that correspondsto write signal. The terms write head, write transducer, write elementand write coil will be used interchangeably herein. The electricalcurrent for the write coil is supplied by write driver 22. The directionof the current flow in the coil, which determines the polarity ofgenerated magnetic field, is typically controlled by four transistorswitches (not shown) in an “H-bridge” arrangement in the write drivercircuit. The spectral content of the write signals tend to have higherfrequency components than the read signals due to the square wave natureof the write voltage (or current) signals generated in the write driver.To achieve high-data rates, low loss high bandwidth transmission linesare used between the write driver and the write elements. In addition,the write transducer's magnetic switching speed can be relatively slowcompared to the desired data rate, which requires a boost or overshootduring write signal reversals. In the interest of avoiding undesirablesignal reflections back to the write element, prior art write driversdeliver current to the coil through transmission lines with static(unswitched) source termination. Write drivers can generally be ofvoltage-type or current-type. Voltage-type write drivers with standardstatically terminated lines are required to operate with voltage supplylevels of the order of 2Z_(o)I_(w), where Z_(o) is the characteristicimpedance of transmission line connecting the write driver to the coil,and I_(w) is the current required by the coil. Current-type writedrivers use shunt-type line termination and therefore need, at launch,to be able to provide current levels of the order of 2I_(w) to achieveI_(w) in the coil.

Designing for low power typically entails lowering the voltage ofoperation. Since the amount of current for proper magnetic recording tobe delivered to the write coil in a magnetic recording systems is fixedby physical constraints in head and media design, lowering the voltageof operation involves lowering the characteristic impedance of the traceinterconnections between the write driver and the write coil. Thisrequires specific design to enable low insertion loss, large bandwidthand adequate physical widths of the low impedance transmission lines.

Prior art design techniques match the write driver's output impedancevalue to be equal or a small percentage greater than the characteristicimpedance of the transmission line. If the write driver's impedance issignificantly mismatched with the characteristic impedance of thetransmission line, undesirable signal reflections can occur in prior artdesigns. The reflected signal can interfere with the transmitted signal,causing distortion and degrading signal integrity. In the prior art theundesirable reflected signal is, therefore, terminated at the writedriver's output, which is also called source-side termination.

FIG. 2 is a conceptual illustration of a prior art current-source-typewrite driver 22 a with current sources 34, 35 and a receiver 24A modeledas simple LRC elements. The write driver 22 a could be used in a diskdrive, for example, and receiver 24A can be a component similar to aninductive write coil in a disk drive. The connections or leads 26C, 26Dto the receiver 24A have characteristic transmission line impedances ofZ₁ and Z₂ which are each one-half of the total Z_(o). Transistorswitches 31 a-d (in the “H-bridge” arrangement) control the direction(polarity) of current flow through the receiver 24A. The transistorswitches are shown in symbolic form to indicate the open (highimpedance) or closed (conducting/low impedance) state. In an H-bridgecircuit, one leg or the other of the bridge is supplying current intothe receiver 24A. In the case of a disk drive, the transition from onepolarity to the other records information in the magnetic media. Asshown, switches 31 b and 31 c are open while 31 a and 31 d are closed toapply current in one direction through the receiver 24A. The state ofthe switches is reversed to apply current in the opposite directionthrough the receiver 24A. In this configuration resistors 29 a, 29 bprovide termination. In a fully differential operation, the node “C” inFIG. 2 behaves as a virtual ground. U.S. Pat. No. 4,414,480 to Zasiodescribes an output circuit designed to take advantage signal reflectionfor a nonterminated line where the receiving circuit appears as an opencircuit to the transmission line. Since the output signal is completelyreflected when it reaches the output end of the transmission line, theamplitude of the signal at the receiving circuit can be double that ofthe initial signal provided by the output circuit. This is due to thereflected signal combining with the incident signal. This cuts the drivecurrent requirements of the output circuit in half. However, if thesignal were to be reflected by the output circuit, this could interferewith the detection of switching transitions. In order to avoid this, theoutput circuit is designed so that its output impedance is approximatelyequal to the characteristic impedance of the transmission line.Therefore, the output circuit provides a series termination for theinput end of the transmission line and will completely absorb thereflected signal.

U.S. Pat. No. 6,671,113 to Klaassen, et al. describes a write drivercircuit that reduces the reversal time for the current through theinductive recording head, The write driver output stage includes asource-side termination circuit having output impedance Z_(S), whereinthe source-side termination circuit output impedance Z_(S) issubstantially equal to Z_(O) and the source strength S_(O) (whichrepresents current drive capability) of the write driver at the input ofthe interconnect circuit is temporarily enlarged after every polarityreversal of the write signal for a predetermined time duration.Klaassen's source-terminated current-type write driver embodiment showsthe write driver source strength enhancement is obtained by connecting ashort current pulse to the input terminals of the integrated leadsuspension (ILS) so that a higher voltage step is created across the ILSinput terminals during a current reversal.

U.S. Pat. No. 6,721,115 to John Price, Jr. describes a technique said toprovide a current boost during the switching transition in acurrent-type write driver by boosting pull-up current during a writecurrent transition.

Note that both U.S. patents above illustrate the state of art, in whichtransition speed is enhanced at the expense of augmenting powerdissipation in the write driver at the transition events.

Data rates in commercial hard disks are expected to move above 3 Gbps inthe next few years, and the continued increase in data rates pose apower and heat challenge for designers. Write drivers are typicallyrequired to provide about 100 mA of current and use 5V power supplies,or approximately 0.5 W of instantaneous power. Since mobile HDDapplications require total power dissipation below 2.5 W includingmotors, etc., there is a need to provide writing capabilities in harddrives at the progressive higher rates at the lowest power levelspossible.

SUMMARY OF THE INVENTION

In this current patent application, an improved low power design isdescribed that uses signal reflections in the transmission lines in adigital signaling system such as a magnetic recording system. Thesereflections are purposefully exploited to provide additional current fora receiver such as the write coil used for magnetic recording. Theinvention allows the write driver to provide less current, thusdiminishing the power dissipation in the write driver with no speedperformance penalty. Even though a magnetic recording system is used asan exemplary embodiment of the invention, those skilled in the art willrecognize that the invention applies to high speed digital signalingsystems where pre-emphasis of the transmitted signal is desired orrequired to provide fast digital signal transitions and compensate thelow pass filtering response of a transmission line interconnect.

An embodiment of the invention uses a transmitter with source-sidetermination and termination controls designed to use signal reflectionand to speed up transition time in written data. Based on the data to bewritten, the transmitter in this embodiment determines the timing,signal levels and which of the electrical connections to thetransmission line will be terminated (or not) and terminated with whatpredetermined impedance levels.

One embodiment of the invention is a digital signaling system thatincludes a transmitter and a remote receiver connected by a transmissionline with at least one transmission line termination resistor withswitching controls for selectively connecting and disconnecting thetransmission line source-side termination resistor. The switchingcontrols dynamically connect and disconnect the one or more resistorsfrom the transmission line so that signal reflections can be exploitedto produce an overshoot or pre-emphasis of the transmitted signal to thereceiver.

Embodiments of the invention include write driver designs using eithervoltage or current sources for use in magnetic storage devices withinductive coil based write heads. Embodiments of the invention usesource-side dynamically switchable terminations in the transmission togenerate signal reflections that constructively add to direct signals toenhance the total current available to drive the receiver such as awrite coil. In one embodiment, as a first signal source of one polarityin the write coil is electrically connected, the terminating impedanceis electrically disconnected for the second signal of the oppositepolarity to generate a transient reflected signal in the write coil thatconstructively adds to the first signal. By controlling and using thereflections in the transmission line that occur when termination isdynamically removed or restored, current-type write driver embodimentsof the invention, for example, can approach the theoretical limit ofdoubling the current at the coil to I_(w), with a writer driver currentsource of I_(w)/2. Thus, the required coil current of approximatelyI_(w) can be achieved using current sources dimensioned to provide onlyI_(w)/2. The invention allows lower instantaneous power dissipation inthe write driver because the reflected signal flows through the writecoil but not through driver voltage or current sources.

The increased current in the receiver is achieved by Dynamic TerminationControl (DTC) of the transmission lines. Electronic switches are used toselectively remove or add termination to the transmission line betweenthe driver power source and the coil. In one embodiment, when the writedriver reverses the polarity of the current flowing through the coil,the configuration of the switches removes the termination for theprevious current path to cause a reflected current pulse to be generatedthat constructively adds to the direct current pulse. This reflectedwave increases the magnitude of the total current pulse in the coil.

In one voltage-type, differential write driver embodiment, twotransistor switches in parallel to terminating resistors are used toswitch the terminating resistors in or out of the circuit to generate atransient reflected signal that constructively adds to a direct signal.

In alternative embodiments of the invention, the duration of the periodwhen the reflected signal is generated can be limited by limiting thetime that terminating impedance is removed from the circuit. In yetother embodiments of the invention, partial termination can be used inconjunction with switching algorithms to further limit and control thetiming and maximum value of the reflected signal as well as theamplitude of the voltage or current the writer driver sends to thereceiver.

In yet another embodiment, simultaneous switching of termination at bothtransmission line terminals at the transmitter end is used. Thisembodiment provides balanced signals in the differential excitation fromwrite driver to write head.

The optimal overshoot amplitude and overshoot duration produced by thecontrolled reflections as well as produced by the write driver itselfare parameters that can be pre-programmed or found by an optimizationprocedure where, for instance, a signaling system using this inventionwould write a specific pattern onto the disk and optimize thoseparameters for minimal bit error rate (BER) in the read data.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a simplified schematic description of selected components in aprior art disk drive including the read and write drivers and the readand write heads.

FIG. 2 is a simplified schematic description of a prior art current-typewrite driver and receiver.

FIG. 3 is a simplified schematic description of an embodiment of theinvention.

FIG. 4( a) is a simplified schematic description of an embodiment of acurrent-type write driver according to the invention with a first switchconfiguration.

FIG. 4( b) is a simplified schematic description of an embodiment of acurrent-type write driver according to the invention with a secondswitch configuration.

FIG. 4( c) is a conceptual schematic description of the switch controlcircuitry in an embodiment according to the invention.

FIG. 5 is a graph illustrating the current flow in the coil according toan embodiment of the invention.

FIG. 6 is a simplified schematic description of an embodiment using avoltage-type write driver according to the invention with a first switchconfiguration.

FIG. 7 is a simplified schematic description of an embodiment using avoltage-type write driver according to the invention with a secondswitch configuration.

FIG. 8 is a simplified schematic description of an embodiment of theinvention using a voltage source based differential write driver withenforced balance of signals.

FIG. 9 is a simplified schematic description of an embodiment of theinvention using a current source based differential write driver withenforced balance of signals.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 3 is a simplified schematic description of a signaling system 22 xembodiment of the invention using a transmitter 28 that includessource-side termination and termination controls 28A and pulse shapingand time control 28B. The transmitter 28 is connected to receiver 24A bytransmission line 27. The decisions on the control of the source-sidetermination are made inside the transmitter based on the input data tobe written and the I/O controls supplied by the read/write channel. Inalternative embodiments of the invention, the overshoot amplitude,duration and the current level the system returns to after the overshootare parameters that can be preprogrammed into the system, can be defined(programmed) by the user at any time, or can be determined by means ofoptimization algorithms that find the optimal value for those parametersto minimize, for instance, bit error rate (BER) in the received data bythe receiver or in the read data from the disk. In the particularembodiment of FIG. 3, the transmitter 28, responding the data input andthe I/O controls determines the timing, signal levels and which of theelectrical connections to the transmission line will be terminated (ornot) and terminated with what impedance levels. The transmitter 28 isdesigned to use signal reflection and to diminish transition time inwritten data.

The transmitter 28 can be implemented using current sources or voltagesources as will be discussed in more detail below, and theimplementation details of the source-side termination will varyaccordingly. The invention controls reflected signal (sometimes called“overshoot”) durations and values by dynamic control of transmissionline termination. Thus, in an embodiment of the invention the overshootduration is not necessarily fixed by the time of flight of the signaltraveling from writer driver to magnetic head. The write driver of theinvention can exploit transient signal reflections to provide additionalcurrent to the receiver (e.g. coil) and therefore save power in thedriver. But, in alternative embodiments the reflections can be stopped(aborted) at a selected time by dynamically re-establishing standardtermination of the transmission line through control of the switches inthe driver. Thus, in alternative embodiments the driver can create thentruncate/abort the signal reflections, shortening the overshoot time toa period that is shorter than the full flight time. The duration of thereflection limits the duration of the overshoot, so in designs where thereflected signal to be exploited is not of sufficient duration the DTCwrite driver returns momentarily to be equal or better, but not worse,than prior art designs.

In addition, the magnitude of the signal reflection can optionally bechanged by tuning the selected impedances, with a corresponding changein the signal source level. An embodiment of the invention can usesignal reflections to further boost a signal that already has someovershoot produced by the driver. The level of overshoot can becontrolled (reduced from its maximum value) by designing the system toinclude intermediate impedances. One alternative embodiment couldprovide a programmable (selectable) option for the termination controlsthat allows programming a fixed impedance mismatch (either above orbelow the characteristic impedance of the transmission line) as aspecial case. Where the transmitter is a current source type driver, thetermination controls can be programmed to terminate the transmissionline with a fixed impedance higher than a characteristic impedance ofthe transmission line (up to an open circuit equivalent). When thetransmitter is a voltage source type driver, the termination controlscan be programmed to leave the transmission line terminated with a fixedimpedance lower than a characteristic impedance of the transmission line(down to a short circuit equivalent). However, using intermediateimpedances rather than open or short circuit impedances, causes someextra power dissipation at the write driver, since some power isdissipated by these selected intermediate impedances. But only in theworst case would this extra dissipation bring a system using thisinvention to dissipate as much extra power to make it equal to prior artsolutions.

The invention can be used with drivers with voltage driven or currentdriven sources, and it can be used with differential or single-endeddesigns. For clarity of illustration the schematics in the figures willbe presented in simplified form. For example, switches will besymbolically illustrated as mechanical switches to show in a simple formthat selected elements or components are selectively connected into thecircuit path or are effectively removed from the circuit path. But thefunction of the conceptual switches can be implemented in various waysaccording to the prior art. For example, where the examples below show aconceptual mechanical-like switch in series with a current source, theswitching function can be implemented using a transistor that iscontrolled to operate in a high impedance state or a negligibly lowimpedance state. But equivalently the current source can be designed tohave a selectable “off state” so that the on-off switch is built intothe current source. Many other switching solutions can be devised usingthe prior art.

A current-driven embodiment will be discussed first. A current-typewrite driver 22 b with Dynamic Termination Control (DTC) according tothe invention is conceptually illustrated in FIG. 4( a). FIG. 4( a) isintended to be a conceptual illustration to aid in explaining theconcepts of the invention rather than a practical implementation. Forexample, as will be obvious to those skilled in the art, a singlecurrent source can be used. Current sources i₁ 34 and i₂ 35 alternatelysupply direct current to write coil 24 through transistor switches 36 a,36 b, 38 a, 38 b. The control lines 36 a′, 36 b′, 38 a′, 38 b′ for theswitches are used to control the on-off state of the switches and areconnected to switch control circuitry such as shown in FIG. 4( c) andwhich will be further discussed below. The control circuitry for theswitches can be implemented using prior art techniques. Although fourcontrol lines are shown, in one alternative embodiment the controlcircuitry reduces these four lines to a single digital control linewhich selects whether the four switches are in the state shown in FIG.4( a) or in the state shown in FIG. 4( b) as will be discussed furtherbelow.

The transistor switches are used to control the direction of currentflow through the write coil 24 and the connection/activity of theterminating resistors 37, 39. When switch 36 a is closed as shown inFIG. 4( a), switch 36 b is open. Thus, in this first embodiment,switches 36 a, 36 b are used to either connect source i₁ 34 or resistor37 to the left side of the write coil 24. The impedance of resistor 37,which is connected to signal ground, is selected to provide transmissionline termination when switch 36 b is closed. Preferably, the impedanceof resistors 37 and 39 are respectively equal to Z₁ and Z₂.

The components of the right side of the write coil 24 are similar tothose described above on the left; however, the switches 38 a, 38 b areoperated in opposite coordination with switches 36 a, 36 b in thisembodiment. Thus, when switch 38 b is open, switch 36 b is closed and soforth. This means that only one of the two termination resistors 37, 39is electrically connected into the circuit at a time. (In an alternativeembodiment described below, the two termination resistors can beconnected at the same time in order to truncate the signal reflection.)When the switch from one configuration state to the other occurs, theeffect is to remove a terminating resistor and generate a transientsignal current reflection that is exploited to produce higher currentlevels in the coil than are provided by the direct current from eithercurrent source. Thus, unlike the continuously terminated lines in thewrite drivers in the prior art, this embodiment of the invention booststhe write signal current in the coil by exploiting reflection on theline caused by dynamic removal of a terminating resistor. The increasedwrite signal current is achieved without drawing additional current fromthe write driver current sources and, therefore, this is a power savingsolution that does not reduce writing transition speed at the writecoil. The effect in the coil is the same for direct current sourced bythe write driver and current produced by use of signal reflections.

The switch configuration in FIG. 4( a), referred to in this document asfirst state for the switches, connects current source i₁ 34 in seriesthrough closed switch 36 a through the coil 24 then through closedswitch 38 b and through resistor 39 to signal ground. In this state, thecurrent source i₁ 34 transmission line is terminated through resistor 39to signal ground. Switch 38 a is open, which disconnects current sourcei₂ 35 from the coil 24. Switch 36 b is also open in this state. Thedirection of direct current from current source i₁ 34 through the coilwill arbitrarily be called “negative,” and the direction of directcurrent from current source i₂ 35 through the coil will be called“positive.”

FIG. 4( b) is a simplified schematic illustration a second switchconfiguration in a write driver 22 b according to the invention. Thefour switches in this second configuration (or state) are reversed fromtheir state in the first configuration (or state) shown in FIG. 4( a).Thus, switch 38 a is now closed and 38 b is open which allows currentfrom source i₂ 35 to flow through coil 24 in the opposite (positive)direction. Switch 36 a is open, which disconnects current source i₁ 34,and switch 36 b is closed to connect resistor 37 (which is connected tosignal ground) into the current path.

The pulse shaping and timing control circuitry 28B for the current-typedriver embodiment of FIGS. 4( a) and 4(b) is illustrated in FIG. 4( c).In this embodiment the standard write signals from the read/writechannel 20 are used as input to the to generate signals for switchcontrols 36 a′, 36 b′, 38 a′, 38 b′. The pulse shaping and timingcontrol circuitry 28B can be implemented using standard logic elements.Pulse shaping and timing control circuitry in other embodiments can havedifferent inputs and a different number of output signals as will bedescribed below. The control signals are a function of the data sequenceto be written (optionally modified by user inputs such as specificovershoot level and duration, or determined by an optimization algorithmthat adjusts overshoot level and duration to minimize BER in the readdata).

The transitions back and forth (toggling) between the positive currentflow and the negative current flow, write magnetic reversals in themagnetic media that are used to encode bits of information. In thisembodiment, the toggling of the write driver is under the control of theread/write channel, which supplies the input data as in the prior art.The invention exploits transmission line reflections in transientlyunterminated lines to produce higher maximum instantaneous currentlevels in the write coil than are provided by either current sources i₁34 or i₂ 35. The operation of the write driver 22 b according to theinvention will be explained with reference to the transition of theswitch states back and forth between the negative configuration of FIG.4( a) and the positive configuration of FIG. 4( b). First assume thatthe negative configuration of FIG. 4( a) has been established longenough for any reflections to have been reduced to negligible levels andthat current from i₁ 34 is flowing into the components as indicated inFIG. 4( a).

FIG. 5 is a simplified graph of the current flowing in the coil whichwill be used in the explanation of the operation of the write driver 22b. The current flow i₁ through the coil as described in the paragraphabove is shown as horizontal segment 54 on the graph. For simplicity ofillustration the current is shown as square waves but the actual waveforms will be rounded, since there is no instantaneous response in realsystems. The signal from current source i₁ 34 is effectively terminatedthrough resistor 39 to signal ground in this state.

Now assume that the switch configuration flips from that of FIG. 4( a)to that of FIG. 4( b), that is from negative to positive. This switchingpoint is labeled as 41 in FIG. 5. In the positive configuration, thecurrent source i₁ 34 is disconnected from the circuit by open switch 36a (that is, transistor switch 36 a is in non-conducting, high impedancemode), but the already launched i₁ current signal traveling from theleft to the right (according to the layouts of the figures) fills bothtransmission lines 32, 33 and will now be momentarily reflected by thehigh impedance provided by both current source i₂ 35, which is connectedthrough closed switch 38 a and open switch 38 b which previouslyconnected resistor 39. The current i₁ injected into the transmissionlines will propagate until properly terminated. When switch 38 b opens,the termination provided by resistor 39 is removed which effectivelyremoves the previous termination. Therefore, the already launched i₁current above will only terminate through resistor 37 which is nowconnected in the circuit by the closed switch 36 b on the opposite sideof the coil. In this way a reflected −i₁ current is transientlygenerated which flows through the coil in the opposite direction (rightto left in FIG. 4( b)). The reflected −i₁ current would be viewed as anundesirable “overshoot” in traditional designs.

At the same time as the reflected current −i₁ is being generated,current source i₂ 35 injects its current i₂ into the transmission linethrough closed switch 38 a. The reflected −i₁ current flows in theopposite direction from the direct i₁ current. Thus, the reflected −i₁current flows through the coil in the same direction as the direct i₂current. The result is that direct i₂ current and the reflected −i₁current through the coil constructively combine (add) and produce aneffective value of:

i _(eff) =|−i ₁ |+i ₂|

for the time period depicted in FIG. 5 as segment 55. The magnitude ofthe current i_(eff) through the coil at this time can be effectivelytwice that which is supplied by either current source.

The effect is transient because the reflected 31 i₁ current eventuallyends after a period equal to the time of signal flight throughtransmission lines 32, 33. The current through the coil then falls tothe i₂ current value which is represented as segment 56 in FIG. 5.

When the write driver switches from the positive configuration to thenegative configuration, the result is similar but of opposite sign. Theswitch from positive to negative is shown as point 42 in FIG. 5. Thecurrent source i₂ 35 is now disconnected with the opening of switch 38 aand current source i₁ 34 is switched into the circuit. The alreadylaunched i₂ current traveling from the right to left (according to thelayouts of the figures) fills both transmission lines, will now bemomentarily reflected by the high impedance provided by both currentsource i₁ 34, which is connected through closed switch 36 a and openswitch 36 b (which previously connected resistor 37) and will onlyterminate through resistor 39 which is now connected by the closedswitch 38 b. At the same time, current source i₁ 34 injects its owncurrent into the transmission lines. The result is that direct i₁current and the reflected −i₂ current through the coil combine (add) andproduce a current:

i _(eff) =|i ₁ |+−i ₂|

for the time period depicted in FIG. 5 as segment 57. The direction ofthe current in segment 57 is opposite to that of segment 55. Themomentarily reflected −i₂ current eventually ends leaving only thedirect i₁ current flowing in the coil as depicted by segment 58. Atswitching point 43 the write driver again switches to the positiveconfiguration and the process repeats as described above.

Programmability of the current levels, both the reflected and direct,can be accomplished by control of the time the transmission lines arekept non-terminated and control of the current levels injected in thelines by current sources i₁ 34 and sources i₂ 35. It should be notedthat whenever the transistor switches open or close, there is apropagation delay before the coil sees any effect produced by thetransition of the switches. This propagation delay is proportional tothe length of the interconnecting components from the write driver tothe coil. FIG. 5 does not show the propagation delay. The switches arethrown every time a change in polarity in the current is required towrite a magnetic transition on the magnetic media, and the current levelat the coil is shown in FIG. 5 as an idealized instantaneous response.Ideally the switches are thrown (opened or closed) simultaneously. Thefinite propagation time for the current signal from writer driver to thecoil is essential for the reflected current to be generated. Thus, theinvention exploits the natural propagation delay on the transmissionline to make the current amplitude reach higher levels when the switchesare thrown resulting in the consequent reversal of current direction andaugment of current level perceived by the coil.

An alternative method of describing the current-type embodiment of theinvention will be presented in the following. The write coil 24 has twoelectrical connections or leads 26A, 26B connected to transmission lines32, 33. The first lead 26A of the write coil and transmission line 32 isswitchably connected (using switches 36 a, 36 b) to either the firstresistor 37, which is in turn connected to signal ground, or the firstcurrent source 34. The second lead 26B of the write coil andtransmission line 33 is selectively connected (using two switches 38 a,38 b) to either the second resistor 39, which is in turn connected tosignal ground, or the second current source 35. In this embodiment thewrite driver is generally maintained in one of two states which will becalled positive and negative. In a first state (negative) the firstcurrent source 34 is connected to the coil to drive a direct currentthrough the coil and into the terminating resistor 39 and then to signalground. In a second state (positive) the second current source 35 isconnected to the coil to drive a direct current through the coil in theopposite direction and into the terminating resistor 37 and then tosignal ground. A magnetic transition is written in the magnetic media inthe storage device by switching the write driver from one state to theother and thereby reversing the direction of current flowing in the coiland generating a reflected current pulse that adds to the direct currentfrom one of the current sources. The write operation, therefore, has twopolarities that correspond to writing either a “1” or a “0” binary bitof information. Thus, one write operation can be described as beginningwith the write driver in the negative state as shown in FIG. 4( a). Thewrite operation then opens switch 36 a, closes switch 36 b, opens switch38 b and closes switch 38 a. These operations are preferably performedapproximately simultaneously.

The reciprocal write operation can be described as beginning with thewrite driver in the positive state as shown in FIG. 4( b). The writeoperation then opens switch 36 b, closes switch 36 a, opens switch 38 aand closes switch 38 b. These operations are preferably performedapproximately simultaneously.

FIG. 6 is a conceptual illustration of an embodiment using avoltage-type write driver 22 v according to the invention with a firstswitch configuration (or switch state). In this embodiment adifferential voltage source type driver 51 provides the power and issupplied input data. Between the voltage source output lines andtransmission lines 32, 33 for the write coil 24 are terminatingresistors 59 a, 59 b in parallel with transistor switches 56, 57 whichhave control lines 56′, 57′. Because switch 56 is electrically parallelto the resistor 59 a, when switch 56 is closed (conducting) as shown ithas the effect of removing terminating resistor 59 a from the circuit.Thus, when switch 56 is open, resistor 59 a acts as transmission linetermination impedance, but switch 56 allows the termination to beremoved under the control of line 56′. Switch 57 and resistor 59 boperate in an analogous way.

FIG. 7 shows the voltage-type write driver 22 v according to theinvention with a second switch configuration (or switch state) in whichswitch 56 is open and switch 57 is closed. In a first embodiment, thewrite driver 22 v operates by toggling between the two switchconfigurations. The functioning of the voltage-type embodiment is inprinciple analogous to that of the current-type described above. Whenthe driver toggles from one switch configuration to the other, thetransient in-flight signal is effectively unterminated and is reflectedso that it constructively adds to the new direct signal generated by thevoltage driver. The current at the write coil would have the same shapeas the current shown in FIG. 5 and would include a transient overshootperiod in which the voltage of the reflected signal constructively addsto the direct voltage signal.

The switching controls for the voltage-type embodiment of FIGS. 6 and 7has only two outputs. In this embodiment the standard write signals fromthe read/write channel 20 are used as input to the switch controlcircuitry to generate signals for switch control lines 56′, 57′. Thecontrol circuitry can be implemented using standard logic elements.

Those skilled in the art will recognize that the switch controlcircuitry could be pre-programmed with knowledge of the time of flightbetween read/write drivers and read/write head, or can be equipped withextra circuits that use a pre-programmed algorithm to determine theflight time once the system is brought to operation.

An alternative embodiment of the invention that truncates the reflectedsignal will now be discussed. In either voltage or current drivenembodiments it is possible that the natural duration of the reflectedsignal at the write coil could exceed that which is needed given thetarget data rate of the storage system. The overshoot created from areflected signal will have a natural duration defined by the “time offlight” Δt_(flight) of the signal (the source outputs) to the write coiland back. In the event that Δt_(flight) is greater than is required forthe target data rate, the overshoot can be effectively truncated byreconnecting the terminating resistor into the circuit. In this familyof embodiments the write drivers will have additional switchconfigurations that will be established by the switch control circuitry.For example, using the voltage source embodiment as a base, analternative that truncated the overshoot period would transition from aconfiguration of FIG. 6 to a configuration of FIG. 7 (or vice versa)and, after some desired overshoot time, would transition to a newconfiguration in which both switches 56, 57 are open. After some time,when there is no signal to be reflected present in the line, one of theswitches would return to its position as depicted in FIG. 7 (or FIG. 6in the case of the transition in the opposite direction), which wouldmake the system using DTC save power by not running current through itssource-side termination associated with the active voltage source. Theswitch control circuitry implementing this embodiment could includetimers that limit the duration that the switches remain closed after avoltage driver reversal. A current-type embodiment that implementedtruncation would similarly have timing logic that would reconnect theterminating resistance after the selected time period, and mightdisconnect it after some desired time interval, when there is no signalon the line that might be usable with exploitation of reflections, whichwould also save power in the write driver.

FIG. 8 is a simplified schematic description of an embodiment of theinvention using a voltage source based differential amplifier writedriver 22 d with enforced balance of signals. Complementary Metal-OxideSemiconductor (CMOS) transistors 71 a, 71 b, 71 c, 71 d are arranged ina differential voltage amplifier configuration and are biased usingcurrent source 64. Termination resistors 79 a, 79 b are connected inparallel with switches 66, 67 respectively and are preferably equal toR₀/2. In the case of a lossless transmission line R₀=Z₀. The samecontrol signal represented by φ₀ is used for the switches 66, 67. Bytying the control lines of switches 66, 67 together, the symmetry andbalance of the signals in the transmission line is enforced in thisembodiment. In this differential implementation, the simultaneousswitching of both termination loads is desirable and makes the signalbalanced. A virtual ground, therefore, appears in the center of symmetryof the circuit. Using differential balanced signals, diminishes thepotential coupling of signaling to other systems whose traces might belaid out in close proximity.

FIG. 9 is a simplified schematic description of an embodiment of theinvention using a current source based differential write driver 22 ewith enforced balance of signals. Bipolar transistors 81 a, 81 b, 81 c,81 d are arranged in a differential current amplifier configuration andare biased using current source 84. Termination resistors 89 a, 89 b areconnected in series with switches 86, 87 respectively and are preferablyequal to R₀/2. In the case of a lossless transmission line R₀=Z₀. Thesame control signal represented by φ₀ is used for the switches 86, 87.By tying the switches 86, 87 together, the symmetry and balance of thesignals in the transmission line is enforced in this embodiment.

The invention has been described in the form of specific embodiments.Alternatives and variations which are apparent or obvious to those ofskill in the art are intended to be within the scope of the invention.Those skilled in the art will recognize that the invention descriptionabove, even though it was presented using its application to storage(hard disk drive) system, can be extended to other high speed digitallinks where transmission line effects can be exploited to produce adesired overshoot of the transmitted signal. In such systems, theovershoot may be called a pre-emphasis of the transmitted signal andmight be used to provide sharper transitions at the receiver side andcompensate for possible bandwidth limitations in the link betweentransmitter and receiver. Those skilled in the art will also recognizethat where the receiver is represented by the resistor “R”, the “L” and“C” elements can be interpreted as parasitic components that might berelated for instance to package parasitics at the receiver end. In thiscase, the invention in this patent application would provide for a lowpower solution generating a pre-emphasis of the transmitted signal bydynamic exploitation of signal reflections.

1. A signaling system comprising: a receiver; a transmission lineconnecting the receiver to a transmitter; and the transmitter, whichsends a signal to the receiver through the transmission line, thetransmitter including source-side termination and termination controlsfor controlling source-side termination to generate signal reflectionsthat enhance the signal.
 2. The signaling system of claim 1 wherein thetermination controls include means for aborting signal reflections aftera controlled period of time.
 3. The signaling system of claim 2 whereinthe means for aborting signal reflections include means for terminatingthe transmission line at the transmitter end with an impedanceapproximately equal to a characteristic impedance of the transmissionline.
 4. The signaling system of claim 1 wherein the terminationcontrols include means for controlling the source-side terminationimpedance level to generate transmission line signal reflections.
 5. Thesignaling system of claim 1 wherein the termination controls include afirst state in which a first impedance approximately equal to acharacteristic impedance of the transmission line terminates thetransmission line and a second state in which a second impedanceterminates the transmission line and produces signal reflections thatincrease the signal at the receiver.
 6. The signaling system of claim 5further comprising means for terminating the signal reflections after aselected period of time has elapsed by terminating the transmission lineat the transmitter end with the characteristic impedance of saidtransmission line.
 7. The signaling system of claim 1 wherein thesource-side termination includes at least first and second selectableimpedances with a first selectable impedance being an impedanceapproximately equal to a characteristic impedance of the transmissionline and a second selectable impedance being a lower impedance than thecharacteristic impedance, which terminates the transmission line andproduces signal reflections that increase the signal at the receiver. 8.The signaling system of claim 1 wherein the source-side terminationincludes at least first and second selectable impedances with a firstselectable impedance being an impedance approximately equal to acharacteristic impedance of the transmission line and a secondselectable impedance, being a higher impedance than the characteristicimpedance, which terminates the transmission line and produces signalreflections that increase the signal at the receiver.
 9. The signalingsystem of claim 1 wherein signaling overshoot amplitudes, overshootdurations or current levels for the system to return to after overshootare programmable.
 10. The signaling system of claim 1 further comprisingmeans for executing an optimization algorithm to determine an optimalsignaling overshoot amplitude, an optimal signaling overshoot durationor a current level for the system to return after overshoot.
 11. Thesignaling system of claim 1 wherein the transmitter is a current sourcetype driver, and where the termination controls are programmed toterminate the transmission line with a fixed impedance higher than acharacteristic impedance of the transmission line.
 12. The signalingsystem of claim 1 wherein the transmitter is a voltage source typedriver, and where the termination controls are programmed to leave thetransmission line terminated with a fixed impedance lower than acharacteristic impedance of the transmission line.
 13. A method ofoperating a signaling system comprising: transmitting a first signal toa receiver through a transmission line terminated with a first sourceimpedance configuration; and switching to a second source impedanceconfiguration to terminate the transmission line, the second sourceimpedance configuration generating signal reflections that enhance thesignal.
 14. The method of claim 13 further comprising aborting thesignal reflections after a controlled period of time after switching toa second source impedance configuration.
 15. The method of claim 13wherein switching to a second source impedance configuration furthercomprises transiently increasing source-side termination impedance to avalue higher than a characteristic impedance of the transmission line.16. The method of claim 13 wherein switching to a second sourceimpedance configuration further comprises transiently decreasingsource-side termination impedance to a value lower than a characteristicimpedance of the transmission line.
 17. The method of claim 13 whereinthe signaling system includes first and second termination resistors inparallel with first and second switches respectively and whereinswitching to the second source impedance configuration further comprisesapproximately simultaneously removing first and second terminationresistors by closing first and second switches.
 18. The method of claim13 wherein the signaling system includes a differential voltage sourcewith first and second source termination resistors in parallel withfirst and second switches respectively, and wherein switching to asecond source impedance further comprises closing the second switch andopening the first switch.
 19. The method of claim 18 further comprisingtransmitting a second signal to the receiver with the second switchclosed and the first switch open.
 20. A method of operating a storagedevice comprising: establishing a first current flowing in a write coilin a first direction with a first driver signal using a first sourceimpedance termination configuration for a transmission line connecting awrite driver and the write coil; and writing a first polarity magnetictransition by switching to a second source impedance terminationconfiguration selected to generate a reflected current flowing in thewrite coil in the second direction that adds to the first current toform an enhanced signal current in the write coil.